1. Field of the Invention
The present invention relates to a bearing state diagnostic apparatus which detects a sound generated from a ball bearing which supports a main shaft of a machine tool, and diagnoses a state of the ball bearing based on a detected value.
2. Description of Related Art Including Information Disclosed Under 37 CFR §§1.97 and 1.98
The bearings are used in rotation devices of many machines. Among the bearings, generally, the ball bearing includes an inner ring, an outer ring, a plurality of rolling elements, and a holder which holds the rolling elements at equal distances from one another, the inner ring rotates together with a shaft, and the outer ring is incorporated in a housing and is fixed. Lubricant oil is charged or supplied into the bearing, and while the main shaft is rotating, an oil film is formed between the inner ring, the outer ring and the rolling elements.
There is no problem when the bearing is normally rotating, but, in the bearing, if rotation failure or seizing occurs due to lubrication failure, invasion of foreign matter, wearing or overload, the machine cannot be operated normally in some cases. To avoid such a problem, it is necessary to pick up and diagnose the state of the bearing.
As an apparatus which picks up and diagnoses a state of the bearing, there are known an apparatus which uses a vibration sensor (see JP-A-2007-10415 for example), an apparatus which uses an ultrasound sensor or a microphone (see JP-A-2005-164314 for example) and an apparatus which uses an AE sensor (see JP-A-2005-62154 for example).
To pick up a state of a bearing, since an S/N ratio is increased, it is preferable that a sensor is mounted directly on the bearing or in the vicinity of the bearing as close as possible. In the apparatus shown in JP-A-2007-10415 in which a state of the bearing is diagnosed using a sensor capable of detecting acceleration, to increase the S/N ration, it is necessary that the sensor is mounted directly on the bearing or incorporated at a position where the sensor is closer to the bearing in the vicinity such as a housing or a spacer which is a part near the bearing. In this case, as compared with a similar constitution machine in which no sensor is incorporated at the time of machine assembling, it is necessary to carryout the wiring processing and to fix the sensor, and the number of assembling steps is generally increased. When the sensor is out of order and it is necessary to replace the sensor by another sensor, there is a possibility that much time must be consumed. Since it is necessary to select a smaller sensor depending on the size or structure of the machine, there are problems that the sensor becomes expensive, satisfactory performance cannot be obtained, the number of kinds of selectable sensors is small. Further, when the machine includes a plurality of bearings, since the same number of sensors as that of the bearings is required, there is a problem that the apparatus becomes expensive, and the constituent parts of the apparatus and machine become complicated. However, if the position where the sensor is mounted is separated from the bearing or from a position near the bearing, and if the sensor is incorporated on an outer periphery or an end surface of the housing, the S/N ratio is reduced although it is easy to exchange the sensor or the constituent parts can be simplified. In this case, it naturally becomes difficult to mount a sensor on each of the bearings, a plurality of bearings are diagnosed using one sensor, and when a machine is constituted by a plurality of bearings having the same or similar specifications, there is a problem that it is difficult to determine which bearing has a problem.
In JP-A-2005-164314, even the apparatus which diagnoses a state of a bearing using a sensor capable of detecting a sound measures variation in a pressure transmitted in the air. Therefore, the position of the sensor is separated away from the bearing, the sensor receives influence of a sound around the machine, a sound generated from other than bearing and an air temperature around the machine, and there is a possibility that the S/N ratio is reduced. Especially in a case of a supersonic sensor whose measurable frequency range is high (20 kHz or higher), many sensors has high directivity, and as the measurable frequency sensitivity becomes higher, the propagation distance of sound becomes shorter. Therefore, it is necessary to mount a sensor near a rolling surface of the bearing.